1. Field of the Invention
The present invention relates to an automotive electric power steering apparatus for applying an assisting force to the steering wheel of a vehicle and, more particularly, to an electric power steering apparatus that provides excellent productivity and reliability.
2. Description of the Related Art
FIG. 14 shows the structure of a general electric power steering apparatus. FIG. 15 is a block diagram of a driver circuit for the electric power steering apparatus. In FIG. 14, a vehicle has a steering wheel 1 that is a steering device. A torque sensor 2 detects the steering torque applied to the steering wheel 1. A vehicle speed sensor 3 detects the speed of the vehicle. An electric motor 4 generates a steering assisting force. A control means 5 receives the output signals from the torque sensor 2 and from the vehicle speed sensor 3 and controls the motor 4 according to the steering torque and the operating conditions of the vehicle to thereby control the value and the direction of the steering assisting torque. A battery 6 is an electric power source for the electric power steering apparatus and supplies a power-supply voltage to the torque sensor 2, vehicle speed sensor 3, motor 4, and control means 5. The circuit configuration of the electric power steering apparatus made up of these components is shown in FIG. 15.
FIG. 15 shows an example of a circuit configuration of an electric power steering apparatus using a brushless motor as the motor 4. An electric motor 4 includes a rotor (not shown) made of a magnet, three-phase armature windings 7 wound around a stator, and an angular position sensor 8 for detecting the angular position of the rotor. A control means 5 includes a microcomputer (CPU) 9, a driver circuit 10, a large capacitor 11 for removing ripples from the electric current flowing through the motor 4, a shunt resistor 12 for detecting the current flowing through the motor 4, a current-detecting means 13 for detecting the current value from a voltage drop developed across the shunt resistor 12, and a switching means 14 for switching on or off the current flowing through the motor 4. The CPU 9 receives the output signals from the torque sensor 2, from the vehicle speed sensor 3, and the angular position sensor 8 and controls the motor 4. The driver circuit 10 controls semiconductor switching elements Q1-Q6 connected in a three-phase bridge form according to a signal from the CPU 9.
In the general electric power steering apparatus of the construction described above, the CPU 9 receives the output signal from the torque sensor 2 that indicates the steering torque. Also, the CPU 9 receives the output signal from the vehicle speed sensor 3 that indicates the speed of the vehicle. The CPU calculates corresponding assisting torque value and driving direction and delivers output signals indicating the calculated values to the driver circuit 10. This driver circuit 10 furnishes pulse width-modulated (PWM) signals to the semiconductor switching elements Q1-Q6 and supplies three-phase alternating power to the motor 4. The CPU 9 receives a current value signal from the current-detecting means 13 and the angular position signal from the angular position sensor 8 to provide feedback. The CPU 9 provides such a control that an appropriate driving current corresponding to both steering torque and vehicle speed is supplied to the motor 4. The circuit of the switching means 14 controlled by the driver circuit 10 is closed normally. The circuit is opened when an abnormality takes place.
In this electric power steering apparatus, the torque sensor 2, the vehicle speed sensor 3, the motor 4, and the battery 6 are installed inevitably in the automotive engine compartment. On the other hand, the control means 5 is normally mounted in the passenger compartment. Therefore, a long wiring harness is necessary to connect them. This wiring harness increases the weight and cost. In addition, the harness leads to power loss and gives rise to radiation noise due to current flowing through the pulse width-modulated motor 4. This radiation noise may malfunction other control devices and induces noise in the automotive radio.
In view of these problems, techniques for eliminating a part of the wiring harness to thereby shorten it have been proposed. In particular, the control means 5 and the motor 4 are integrated as a subassembly, which in turn is mounted in the engine compartment. For example, in the technique disclosed in Japanese laid-open patent No. 30434/1997, a metal case is interposed between a DC brush motor and a speed-reduction mechanism for reducing the speed of rotation of the motor. A single metal substrate or board is disposed in this metal case. The control means 5, the semiconductor switching elements Q1-Q6, and the brush holder of the motor are mounted on this board. This technique disclosed in this Japanese laid-open patent reduces radiation noise and takes account of heat dissipation. However, the motor itself is inevitably made bulky. Especially, it has an extremely large radial dimension. This deteriorates the ease with which the motor is mounted to the steering mechanism. During mounting operation, the spaces assigned to surrounding parts are inevitably sacrificed.
Furthermore, Japanese laid-open patent No. 192757/1996 discloses a technique having a metal case mounted between a brush DC motor and a speed-reduction mechanism that reduces the rotational speed of a motor. Semiconductor switching elements Q1-Q6 and the brush holder of the motor are received in this case. A casing that houses a control means 5 is mounted radially adjacent to the metal case. In the same way as the previously mentioned known technique, large radial space is necessary. The motor is not easily mounted to the steering mechanism. In addition, the electric power steering apparatus itself is fabricated with decreased productivity.
Additionally, Japanese laid-open patent No. 117093/1997 discloses a technique for use with an electric power steering apparatus using a brush DC motor. This technique uses a substrate or board on which a control means 5 and switching elements Q1-Q6 are mounted. This board is held to an external case on which the brush holder of an electric motor is mounted. The external case holding the board is located at a side of the motor within the housing of the motor. This technique permits simplification of wiring leads among components. Also, increase in size can be suppressed to some extent. However, the semiconductor switching elements are located beside the motor that produces heat. In addition, the switching elements are mounted within the housing of the motor. Therefore, the thermal reliability presents problems. Furthermore, the current-carrying capacity of the semiconductor switching elements is limited. Additionally, limitations are placed on the increase of the output of the motor. Also, the productivity poses problems.
The present invention has been made to solve the foregoing problems. It is an object of the invention to provide an electric power steering apparatus which reduces radiation noise due to PWM drive and permits higher output power, improvement of productivity, and weight reduction.
An electric power steering apparatus in accordance with the present invention comprises:
an electric motor having an output shaft;
a speed-reduction mechanism for reducing rotational speed of said output shaft of said motor;
a power circuit board having power circuit parts mounted thereon, said power circuit parts including semiconductor switching elements for supplying electric power to said electric motor;
a control circuit board having general circuit parts generating little heat, said general circuit parts including a microcomputer for controlling said semiconductor switching elements according to an output signal from a torque sensor for measuring steering torque and according to an output signal from a vehicle speed sensor for measuring vehicle speed;
a circuit case body including an insert-molded wiring conductors and having a first connector adapted to connect with said torque sensor, a second connector adapted to connect said vehicle sensor and a third connector adapted to receive an electric power supplied from outside, said first, second and third connectors are integrally molded with said circuit case body;
a heat sink for dissipating heat from said power circuit parts on said power circuit board, said heat sink being mounted to said speed-reduction mechanism;
a controller formed by said power circuit board, said control circuit board and said circuit case body;
wherein said power circuit board and said control circuit board being located opposite to each other along the axis of said output shaft of said electric motor within said circuit case body of said controller; and
said electric motor and said heat sink are combined in such a way that said controller is sandwiched between said electric motor and said heat sink.
According this structure, the controller and the motor can be assembled and tested separately. Then, they can be integrated. This secures high productivity and reliability. External wire leads can be shortened. Power loss and radiation noise are decreased. Furthermore, only components consuming only small amounts of current are packed on the control circuit board. Components consuming large amounts of current are packed on the power circuit board. Heat generated by the power circuit board and by the motor is dissipated through both heat sink and speed-reduction mechanism. Consequently, a small-sized, electric power steering apparatus having excellent heatproofness and capable of coping with increase in the output of the motor can be obtained.
In one feature of the electric power steering apparatus according with this invention, said heat sink and said speed-reduction mechanism have their respective fitting portions fitted together and mounted coaxially, and wherein said electric motor and said heat sink are combined coaxially.
According this feature, rotation of the motor is transmitted smoothly to the worm gear. In consequence, noise and vibrations can be reduced.
In another feature of the electric power steering apparatus according with this invention,
(A) said first, second and third connectors are arranged in a line substantially perpendicularly to the axis of said electric motor,
(B) said first connector is located between said second and third connectors, and
(C) external leads to be connected with said second and third connectors, respectively, are connected in a direction substantially parallel to the axis of said output shaft of said electric motor at outside of said electric motor.
According this feature, during assembly of the vehicle, wiring operation can be easily performed. The space occupied by the wiring can be reduced.
In further feature of the electric power steering apparatus according with this invention, said heat sink, said power circuit board, and said control circuit board are provided with shaft holes through which said output shaft of said electric motor passes, and wherein the shaft holes in said heat sink and in said power circuit board are set larger in diameter than the shaft hole in said control circuit board.
According this feature, during assembly using a jig, this structure prevents the jig from interfering with the output shaft holes in the heat sink and in the power board. Hence, deformation of the power board and solder cracks can be prevented.
In a still other feature of the electric power steering apparatus according with this invention, the electric power steering apparatus further comprises an angular position sensor to measure the angular position of said electric motor, said angular position sensor includes;
(A) a permanent magnet magnetized into multiple poles mounted on said output shaft of said electric motor, and
(B) a magnetic detecting device disposed on said control circuit board.
According this feature, the electric power steering apparatus dispenses with connectors and wire leads that would normally be used to connect the magnetic sensor with the control board. As a consequence, malfunctions due to noise transmitted through the leads can be prevented.
In an additional feature of the electric power steering apparatus according with this invention, a capacitor for absorbing current ripples flowing through said electric motor is mounted on a peripheral portion of said power circuit board and located in a cutout portion formed in said control circuit board.
According this feature, the space between the power board and the control board can be reduced. This leads to miniaturization of the power steering apparatus.
In a still other feature of the electric power steering apparatus according with this invention, wherein
(A) said electric motor includes motor winding terminals extending from said electric motor toward said heat sink,
(B) said insert-molded wiring conductors includes winding terminals connected with said motor winding terminals at the inner side of the fitting portions of said heat sink and said speed-reduction mechanism fitted together.
According this feature, a cover is mounted on the inner side of the fitting portion of the heat sink. The connected portions can be protected against protrusion of foreign materials.
In a yet additional feature of the electric power steering apparatus according with this invention, the electric power steering apparatus further comprises a current detecting means is mounted on said control circuit board near a position at which said winding terminals connected with said motor winding terminals, and wherein an electrical current flowing through said electric motor is detected, based on the current flowing through said motor winding terminal.
According this feature, wire leads for detecting an electrical current are dispensed with. Consequently, the size of the power steering system can be reduced.
In a yet further feature of the electric power steering apparatus according with this invention, said motor winding terminals includes a U-shaped portion, and said current detecting means includes a magnetic current detecting device disposed on said control circuit board at the one side of said U-shaped portion, and a magnetic collector made of a ferromagnetic material disposed on opposite side of the U-shaped portion.
According this feature, the magnetic flux can be concentrated effectively. Consequently, the accuracy at which an electrical current is measured can be improved.
In a still further feature of the electric power steering apparatus according with this invention, said current detecting means includes a C-shaped magnetic collector made of a ferromagnetic material forming a opening between its both opening ends and a magnetic current detecting device mounted on said control circuit board within said opening of said C-shaped magnetic collector, and wherein said motor winding terminals includes a U-shaped portion magnetic coupled with said C-shaped magnetic collector.
According this feature, the magnetic flux can be effectively concentrated. The accuracy at which an electrical current is measured can be enhanced.
Other objects and features of the invention will appear in the course of the description thereof, which follows.